Acoustic enclosure for loudspeakers



Dec. 15, 1959l L.. J. ELLIOTT 2,917,127

ACOUSTIC ENCLOSURE FOR LOUDSPEAKERS Filed Feb. 1o, 195e 5" |56- e` -efc:gio o o o o /fl g ||io o ooo 4 51H0 o o o o VA 1 lUV T n *02 2d:- 6a jQ--f 1 5 I f I 3 r// I m E 2 7 E a 9. `5

nvenlor l A Harney United States Patent O 2,917,127 ACOUSTIC ENCLOSUREFOR LOUDSPEAKERS Llewellyn .lohn Elliott, Cambridge, England, assignorto Pye Limited, Cambridge, England, a British company ApplicationFebruary 10, 1956, Serial No. 564,809

Claims priority, application Great Britain February 17, 1955 3 Claims.(Cl. 181-'31) The useof an enclosure inthe form of a Helmholtz resonatortuned to the free air resonance of a loudspeaker unit is well known inthe art. v Such enclosures are generally known as bass-reflex or phaseinverter cabinets. n

Y `A disadvantage of such a system is that although the main resonanceof the speaker is effectively damped and the bass response extended, twosubsidiary resonances are formed. In one themass and air loading of themoving parts of the loudspeaker unit resonate with the acousticcapacitance of the air enclosed by the cabinet and in the other themechanical stiffness of the unit resonates with the acoustic inductanceof the port aperture of the resonator. As a result the frequencyresponse of the system is non-linear in the region of these unwantedresonances.

It is an object of the present invention to reduce the amplitude ofthese unwanted resonances with a resulting smoother frequency responseand consequent reduction in distortion of the reproduced sounds.

According to the invention, one or more internal Helmholtz resonatorsarel introduced into a reex enclosure adapted to contain a loudspeakerunit and are tuned to the resonance frequency of the loudspeaker unitwith the acoustic capacitance of the air enclosed by the enclosure or tofrequencies in va band including that resonance.

According to a feature of the invention, a further subsidiary reflexsystem or systems are arranged to be coupled acoustically to the mainport aperture or apertures of the main rellex system constituted by theenclosure and are tuned to the resonance of the loudspeaker unit withthe acoustic inductance of the main port aperture or apertures or tofrequencies in a band including that resonance.

The enclosure is constructed so as to be mechanically .rigid and linedwith suitable sound absorbing material. If necessary membranes of asound absorbing material such as felt are mounted across one or all ofthe internal chambers to absorb standing waves formed by reflectionsbetween the walls or partitions in the enclosure.

The frequency response of the system is considerably smoother and isextended further towards the lowest acoustic frequencies than that of aconventional recx system occupying the same volume and fitted with asimilar loudspeaker uni-t.

In order that the invention may be more fully understood reference willnow be made to the accompanying drawing in which:

Figure l is a front elevation of a loudspeaker enclosure according tothe invention with the decorative fret removed, and

Figure 2 is a section taken on the line II-II in Figure 1.

Referring to the drawing, the enclosure comprises an outer casing 1 ofgenerally rectangular form and preferably made of wood, for exampleplywood and constructed so that it is mechanically rigid. The side ofthe casing 1 which is intended to form the front, is closed by aloudspeaker board 2, n front on which is arranged a decora,-

ice

tive fret 3 which may be a sheet of expanded metal, and is retained inposition by means of a detachable framework or beading 4. The interiorof the cabinet is divided horizontally by means of a sloping shelf 5,which extends between the two opposite side walls of the casing 1. Bymaking the shelf 5 sloping, the possibility of producing standing waveswithin the cabinet is reduced. The space above the shelf 5 is dividedvertically by a partition 6 extending between the upper surface of theshelf and the top of the casing 1 in order to form two compartments,namely a rear compartment A and a front compartment B. The bottom of thefront compartment B is formed by a horizontal shelf 7, which defines,with the shelf 5, 'a third compartment C below the compartment B. Theshelf 7 is provided with a plurality of apertures 7a y which providecommunication between the compartment B and the compartment C. Thespeaker board 2 is also and which is mounted with its cone over theaperture 2a in the speaker board 2 and, the rear of the loudspeakerextending through an aperture 6a in the partition 6.

The enclosure A and part of the enclosure B are lined with asound-absorbing material 10, for example a layer of cotton waste coveredwith thick felt.

In operation, the main port apertures 7a resonate with the volumes ofthe compartments A and B at free air resonance of the loudspeaker unitas in a conventional reflex system. According to the invention, theaperture 6a forms anannular air gap between the rear of the loudspeakerand the side walls of the partition 6 which resonates with the volume ofcompartment A as a Helmholtz resonator and reduces the amplitude of theresonance of the speaker unit with the volumes A and B. Thus thisinternal Helmholtz resonator is tuned to the resonance frequency of theloudspeaker unit with the acoustic capacitance of the air enclosed bythe cabinet or to frequencies in a band including that resonance.

Furthermore, the apertures 8 resonate with the volume of air in thecompartment C as a subsidiary reilex system and reduces the amplitude ofresonance of the speaker unit with the main port apertures 7a. Thus thissubsidiary reex system is coupled acoustically to the port apertures 7aof the main reflex system and is tuned to the resonance of theloudspeaker unit with the acoustic inductance of the main port apertures7a or to frequencies in a band including that resonance. The size andspacing of the holes 8 is important to give the desired resonancefrequency.

In this embodiment of the invention, the space beneath the shelf 5 formsa compartment D which may be emlployed to house an amplifier to be usedin conjunction with the loudspeaker. One end of this space may bedivided by means of a wall 11, to form a subsidiary compartment E whichmay house an electrical cross-over unit. Ventilation holes 12 are formedin the front, rear and bottom of the amplifier compartment D, andpreferably the rear of the compartment is provided with a removable door13, in order to give access to the interior Aof the cornpartment. Theloudspeaker enclosure may be mounted on castors 14 so that it can bereadily moved from place to place.

Whilst a particular embodiment has been described, it will be understoodthat various modifications may be made without departing from the scopeof this invention. For example the cabinet may be constructed withoutthe bottom compartments intended to house the amplifier and cross-overunit.

I claim:

1. An acoustic enclosure comprising a substantially 3 rectangular casinghaving a front wall, a rear wall, two side walls, a top and a bottomwhich slopes from one side to the other across the enclosure, aninternal substantially vertical partition extending from side to sideacross the casing and dividing the internal space into a frontcompartment and a rear compartment which together form a bass-reexsystem, a loudspeaker unitarranged in said front compartment, anaperture ,in .said front wall for the egress of sound from theloudspeaker unit, an aperture in said partition through which the rearof the loudspeaker projects, and a space between the rear of saidloudspeaker and the side wall of said aperture forming an air gap aroundthe rear portion of the loudspeaker unit which is arranged to resonatewith the volume of air in the rear compartment as a Helmholtz resonatorand thereby reduce the amplitude of the resonance kof the loudspeakerunit with the volume of air enclosed in the casing.

2. An acoustic enclosure comprising a substantially rectangular casinghaving a front wall, a rear wall, two side walls, a top and a bottomwhich slopes from one side yto the other across the enclosure, aninternal substantially vertical partition extending from side to sideacross the casing and dividing the internal space into a frontcompartment and a rear compartment which together form a bass-reilexsystem, a loudspeaker unit arranged in said front compartment, anaperture in said front wall for the egress of sound from the loudspeakerunit, an aperture in said partition through which the rear of theloudspeaker projects, and a space between the rear of said loudspeakerand the side wall of said aperture forming an air gap around the rearportion of the loudspeaker unit which is arranged to resonate with thevolume of air in the rear compartment as a Helmholtz resonator andthereby reduce the amplitude of the resonance of the loudspeaker unitwith the volume of air enclosed in the casing, and a membrane of soundabsorbing material mounted across at least one of the compartments toabsorb standing waves.

3. An acoustic enclosure comprising a substantially lrectangular casinghaving a front wall, a rear wall, two side walls, a top and a bottom, aninternal substantially vertical partition extending from side to sideacross the casing and dividing the internal space into a frontcompartment and a rear compartment which together form a bass-retiexsystem, a loudspeaker unit arranged in said front compartment, anaperture in said front wall for the egress of sound from the loudspeakerunit, an aperture in said partition through which the rear of theloudspeaker projects, and a space between the rear of said loudspeakerand the side wall of said aperture forming an air gap around the rearportion of the loudspeaker unit which is arranged to resonate with thevolume of air in the rear compartment as a Helmholtz resonator andthereby reduce the amplitude of the resonance of the loudspeaker unitwith the volume of air enclosed in the casing, a horizontal partitionarranged belowsaid loudspeaker unit and forming the bottom of said frontcompartment, a lower compartment beneath said horizontal partition, atleast one aperture between said lower compartment and the outside of thecabinet, at least one aperture in the horipartment and the outside ofthe cabinet resonating with the volume of air in the lower compartmentas a subsidiary reflex system and reducing the amplitude of resonance-of the speaker unit with the at least one aperture in the horizontalpartition.

References Cited in the le of this patent UNITED STATES PATENTS1,804,676 Dalton May 12, 1931 l1,837,755 Carlisle et al. Dec. 22, 19311,869,178 Thuras July 26, 1932 1,969,704 DAlton Aug. 7, 1934 2,020,166Swinyard Nov. 5, 1935 2,179,840 Bucky Nov. 14, 1939 `2,642,948 Olson etal June 23, 1953 2,694,462 Robbins et al Nov. 16, 1954 2,766,839 Baruchet al Oct. 16, 1956 FOREIGN PATENTS 434,563 Great Britain Sept. 4, 1935489,588 Great Britain July 29, 1938 496,487 Great Britain Nov. 28, 1938663,734 Great Britain Dec. 27, 1951 696,671 Great Britain Sept. 9, 1953

